This invention relates to apparatus for and method (process) of transferring heat, and, specifically, relates to such apparatus and method for the isothermal transfer of large quantities of heat within a closed container.
Generally, this invention relates to heat exchangers and heat transport systems, and, more particularly, to change-of-state closed cycle heat transfer systems. Apparatus of this invention includes a closed container having, for example, tubes passing therethrough for carrying high and low temperature materials so as to serve, respectively, as a heat source and a heat sink or heat absorber. Such tubes are disposed within the container so as to be in heat exchange relation with one another by means of an intermediate working fluid which is alternately evaporated on the heat source tubes and condensed on the heat absorber tubes.
Heat exchangers generally can be divided into two main groups--conduction heat exchangers and change-of-state heat exchangers. Change-of-state heat exchangers have been known in the art for considerable time in the form of steam boilers, steam heating systems, vaporizers, thermo-syphons, vapor chambers, and the heat pipe. Of the various means of transmitting heat, the heat pipe is, in many respects, the most efficient and satisfactory. However, problems have been associated with the heat pipe in the design and development of a practical change-of-state vapor heat exchanger utilizing heat pipe principles.
To review briefly the history of the heat pipe, a closed cycle heat transfer system similar to a heat pipe was first disclosed in British Pat. No. 22,272 granted to Perkins et al on Dec. 5, 1892. In U.S. Pat. No. 1,725,906 granted to Frazer W. Gay on Aug. 27, 1929, a principal object of Gay's invention was to provide a form of heat transfer wherein a closed tubular element is provided within one end of which is a volatile liquid which, under the influence of heat conducted thereto through this one end, is caused to boil and to yield a hot vapor which rises to the opposite end of the tube at which point the vapor condenses. Then, the condensate moves back to the lower end of the tube under the influence of gravity. It is noted, however, that the lower end of the tube disclosed by Gay has a liquid level and, therefore, the device is more accurately classified as a thermo-syphon rather than a heat pipe.
In U.S. Pat. No. 2,350,348 granted to R. S. Gaugler on June 6, 1944, a device was disclosed in which a capillary element was placed within a pipe for the purpose of conducting or transporting liquid condensate back to the evaporator from the condensing section of the pipe even against the pull of gravity. Thus, Gaugler disclosed a method of transferring heat from a higher temperature to a lower temperature in any desired direction by evaporating a volatile liquid at a first point at a higher temperature, conveying the vapor to a lower temperature point, condensing the vapor on the lower temperature point, and returning, by capillary action, the condensed liquid back to the higher temperature point.
In U.S. Pat. No. 3,229,759 issued Jan. 18, 1966 to G. M. Grover, an evaporation-condensing condensation heat transfer device was disclosed which provided a thermal conductivity greatly exceeding that of any known metal by a large factor. Grover coined the term "heat pipe" to note a heat transfer device which within a closed tube or pipe, a liquid was evaporated in an evaporator section of the pipe, the vapor was transported through the pipe, and the vapor was condensed in a condenser section of the pipe with return of the condensate to the evaporator section being accomplished by a wick.
In its simplist form, a heat pipe is essentially a closed, evacuated chamber with a volatile liquid therewithin. At one end of the chamber, the evaporator section, the liquid is heating thereby causing it to vaporize. The resulting pressure difference between the evaporator section and the cooler end, referred to as the condenser section, forces the vapor (and thus the heat energy contained in the vapor) to move toward the condenser section. When the vapor reaches the condenser section, it encounters a lower temperature surface than that of the evaporator section (i.e., a temperature lower than its boiling temperature at the pressure within the container). As a consequence, the vapor condenses on the condenser section thereby releasing the energy stored therein (i.e., releasing the latent heat of vaporization). Once the vapor has condensed, the liquid condensate is returned to the evaporator section to complete the cycle.
It is important to note that the vapor stores heat energy at the temperature at which the liquid was evaporated, and that the vapor will retain that same temperature (and energy) until it meets a colder surface and condenses. This results in the entire chamber of the heat pipe remaining at a constant temperature (i.e., to be isothermal), and is responsible for the high thermal conductance properties of heat pipes.
The choice of composition and structure of the capillary materials used in the heat pipe for return of the liquid condensate from the condenser to the evaporator is dependent upon its compatibility with the working fluid and upon the working temperature of the heat pipe. Some known examples of capillary materials include copper, nickel, and aluminum porous or woven materials. In addition, certain ceramic fibrous materials may be used.
The working fluid used in a heat pipe is the actual heat transfer medium. The choice of heat transfer medium depends, to a large extent on the intended operating temperatures of the heat pipe. Generally, the medium must have a melting point below and a critical point above the heat pipe operating temperature. Such operating temperatures can be divided into a cryogenic range (i.e., less than 122.degree. K.) in which liquid gases, such as nitrogen are used; moderate temperature ranges (122.degree.-628.degree. K.) in which materials such as methanol or water are used; and the liquid metal range (at temperatures greater than 620.degree. K.) in which liquid metals such as potassium, lithium or sodium are used.
Generally, as the amount of surplus liquid in a heat pipe diminishes, the performance of the heat pipe improves dramatically. The heat pipe disclosed in Grover would appear to be the first true heat pipe because the quantity of working fluid is selected so that no surplus liquid is provided at the desired operating temperature of the heat pipe.
Reference may also be made to U.S. Pat. No. 3,613,779 to Clinton E. Brown in which it is stated that heat transfer in a liquid evaporation or condensation system is generally limited by the low thermal conductivity of the fluid relative to the thermal conductivity of the heat exchange surfaces of the system. Because the heat flux of a heat exchange surface is virtually proportional to the thickness of the fluid on the surface, the average thickness of the heat pipe flowpath through the fluid must be held to a minimum in order to maximize the heat transfer rate.
It is also generally recognized that in work producing thermodynamic or heat transfer processes, the ideal process is reversible. The measurement of the efficiency of a practical device or process is how close it comes to this ideal thermal reversible process. In thermodynamics, the closest approach to true thermal reversibility is the process of a vapor being condensed on a cold wall. In the cyclic heat pipe process of evaporation and condensation, the entire cycle becomes nearly reversible if the evaporation process substantially matches the reversibility of the condensation process.
It is also generally known that stable film evaporation can remove the heat from a surface at a rate several orders of magnitude larger than nucleate boiling or surface pool boiling. In high performance heat pipes, there is no nucleate boiling on the evaporator and the heat pipe is designed so that all heat transfer is stable film evaporation thereby to take advantage of the high heat flux properties of stable film evaporation.
As taught by Grover in his U.S. Pat. No. 4,020,898, the heat pipe is theoretically capable of transferring heat at much greater rates than conventional heat transfer systems because it operates on the principle of phase change rather than on the principle of conduction or convection. Grover also noted a number of difficulties had been experienced in attempting to use heat pipes in commercial applications. For example, Grover pointed out that heat pipes actually in operation typically utilized a capillary wick to transport the liquid longitudinally in the heat pipe from the evaporator section to the condenser section. In heat pipes using a wick, the quantity of working fluid is selected so that no surplus liquid is provided at the desired operating temperature. As a result there is only modest interference between the liquid and vapor phases of the working medium. However, capillary wicks were recognized to be difficult and expensive to install and for this reason the use of heat pipes incorporating such wicks has been limited to special and expensive application such as a nuclear reactors in spacecraft.
Attempts have been made to utilize the heat pipe principles in a single unit containing a heat source and heat absorber. U.S. Pat. No. 3,986,340 issued to Henry W. Bivens, Jr. in 1976, a closed system for gasifying liquid natural gas was disclosed and this system was based on the principle of heat transfer as used in heat pipes. However, this system was dependent upon gravity to return the condensate to a pool of working fluid in which the heat source was immersed. Vaporization took place at the surface of the pool of working fluid (as opposed to the surface of the heat source) and required that the entire pool of liquid be raised to its boiling temperature.
In the heat pipe, movement of the vapor is responsible for transporting heat energy from the evaporator section to the condenser section. This principle is, of course, the same as that used in conventional steam heating systems. What distingushes the heat pipe from such systems is not the capillary means which returns the liquid to the evaporator, but rather the stable film process on the surfaces of the evaporator. A stable film evaporation process combined with a stable film (or drop-wise) condensation process from a thermodynamic viewpoint, is nearly ideal for the fast and efficient transfer of heat. In his paper entitled "Heat Pipe" published in the May, 1968 "Scientific American," G. Yale Eastman set out five important properties of heat pipes which are as follows:
First, heat pipes operate on the principle of vapor heat transfer and can have several thousand times the transfer capacity of metallic conductors.
Secondly, heat pipes exhibit a property called "temperature flattening". There are many heat transfer applications in which a uniform temperature over a large surface area is required. A heat pipe can be coupled to a non-uniform heat source so as to produce a uniform temperature at the output, regardless of the point-to-point variations of the heat source.
Thirdly, the evaporation and condensation functions of a heat pipe are essentially independent operations connected only by the flow of vapor and liquid within the container. The patterns and areas of evaporation and condensation are independent. Thus, the process occurring at one end of the heat pipe can take place uniformly or non-uniformly over large or small surface areas, without significantly influencing the process occurring at the opposite end of the heat pipe. This separation of functions leads to one of the most valuable properties of the heat pipe--its ability to concentrate or to disperse heat. This property has been called "heat flux transformation".
Fourthly, heat pipes have exhibited a property that makes it possible to separate the heat source from the heat sink. It is often inconvenient or undesirable to have the heat source and the heat sink in close contact within practical thermodynamic processes.
Lastly, heat pipes also can be operated so that the thermal energy and/or the temperature at which the energy is delivered to the intended heat sink be held constant in spite of large variations in the energy input to the heat pipe. This surplus power beyond the needs of the heat sink can be dissipated by means of an excess power radiator or condenser.
Generally, limitations of heat pipe-based technology have been due to design. Heat pipes are generally point-to-point heat transfer apparatus as contrasted with area-to-area apparatus. A number of individual prior art heat pipes have combined in a bundle so as to result in an apparatus which can be effectively used to transfer heat in volume. In these prior art heat pipe designs, however, the ends of the individual heat pipes often times project into the heat source and heat sink fluid thus creating restriction and turbulence which restricts the flow of these fluids. Further, in conventional heat pipe designs, the evaporator surfaces and condenser surfaces share their operating areas with the walls of the heat pipe structure. To increase or decrease the functional or operating areas of the evaporator and condenser sections, the wall structure of the heat pipe has to be expanded or otherwise distorted. Further, in a heat pipe, the energy recovered from the heat source is a function of the area of the evaporator surface on the wall of the heat pipe and the heat delivered is a function of the volume of the interior of the heat pipe. In expanding the size of the heat pipe container, the inside volume increases at a rate much faster than that of the wall area. It would be of no real value to expand the size of the heat pipe without means for increasing the evaporator area.
Heretofore, there have been many formidable barriers and severe limitations inherent in prior art heat pipe designs which are overcome by the apparatus and method of the present invention as will be hereinafter described in detail.
Reference may be made to such other U.S. patents as U.S. Pat. Nos. 2,119,091, and 2,919,551 in the same general field as the instant invention.